EF 400mm f/5.6 L - wide open vs. f/8

[Mark Itol]

My field observation on my copy of the EF 400mm f/5.6 L has been leading me to believe that it is as sharp (visually) wide open (f/5.6) as it is when stopped down to f/8 (albeit the MTF charts indicate an improvement in both contrast and resolution when stopped down to f/8). I conducted an experiment last August (but the results got buried in other stuff) and I'm posting below my findings.

Parameters:
- Canon EOS 40D (firmware 1.1.1), Manfrotto 190XB + 393 Gimbal Head
- ISO 400, Auto WB, Av Priority AE, +2/3 EC, RAW
- Live View (mode 2), Manual Focus at 10x magnification, Cable Release
- Subject at lens MFD of ~3.5 meters, Focused on the left eye
- Neutral Picture Style (sharpness: 0, contrast: 0, saturation: 0, color tone: 0)
- DPP 3.8.1.0 (default settings), No adjustments except for cropping and conversion to quality 10 jpeg
- Images were shot in burst and the "best" (judged visually and based on file size) shots were taken for comparison
- The images were shot outdoors and were shot less than a minute apart (to minimize the possibility of a change in lighting)

Results:

f/5.6, 1/640 sec


f/8, 1/320 sec


What do you guys think? My eyes can't tell the difference. It is interesting that the image shot at f/5.6 has a larger size compared to the one shot at f/8. If this test confirms my observation, I'll be happily shooting wide open and will only stop down for more DOF or to help tame CA when shooting backlit subjects.

Suggestions regarding my procedure are also welcome.

Other 400 f/5.6 shooters, let's post our tests here!

Link to Test

[Bobby Kintanar]

I'll be re-testing my old workhorse again soon. I've noticed that it would back-focus sometimes, and even sent it Manila thru Ka Romy. I think I know why it was "erratic" now. The cause was my TC or if I use a Sigma Lens, then replace it with the Canon, while the unit is "ON" it would Back-focus. Now, whenever I use a TC with it, I always turn it off before replacing the lens!.

Here are some shots I took with it after it came back from Canon, Manila, all at f/5.6 to f/6.3 using the EOS 60D:







Will do the tests when I have time.

[gabriel buluran]

try it with the new 500 peso bill mark.